storpool: fix pom.xml which caused docker image build failures (#7845)

steps to reproduce the issue:
- git clone https://github.com/apache/cloudstack.git
- cd cloudstack
- rm -rf .git/
- run `mvn -P developer,systemvm clean install`

Without this PR, it fails with error

```
 > [ 8/10] RUN mvn -Pdeveloper -Dsimulator -DskipTests clean install:
668.1 [ERROR] Failed to execute goal pl.project13.maven:git-commit-id-plugin:4.9.10:revision (get-the-git-infos) on project cloud-plugin-storage-volume-storpool: .git directory is not found! Please specify a valid [dotGitDirectory] in your pom.xml -> [Help 1]
```

plugins/storage/volume/storpool/pom.xml

@@ -69,6 +69,31 @@
     </dependencies>
     <build>
         <plugins>
+            <plugin>
+                <groupId>org.codehaus.gmaven</groupId>
+                <artifactId>gmaven-plugin</artifactId>
+                <version>1.5</version>
+                <executions>
+                    <execution>
+                        <id>set-properties</id>
+                        <phase>validate</phase>
+                        <goals>
+                            <goal>execute</goal>
+                        </goals>
+                        <configuration>
+                            <providerSelection>1.7</providerSelection>
+                            <source>
+                                File git = new File("./.git")
+                                if (!git.exists()) {
+                                    pom.properties['storpool.skip.git.properties'] = 'true'
+                                } else {
+                                    pom.properties['storpool.skip.git.properties'] = 'false'
+                                }
+                            </source>
+                        </configuration>
+                    </execution>
+                </executions>
+            </plugin>
             <plugin>
                 <artifactId>maven-surefire-plugin</artifactId>
                 <configuration>
@@ -96,6 +121,7 @@
                     </execution>
                 </executions>
                 <configuration>
+                    <skip>${storpool.skip.git.properties}</skip>
                     <dotGitDirectory>${project.basedir}/.git</dotGitDirectory>
                     <prefix>git</prefix>
                     <verbose>false</verbose>